The development of modern rubber materials has made possible the manufacture of a wide range of elastomeric articles having varying properties of strength and chemical resistance. As synthetic latex materials have developed, various elastic and polymeric materials have been adapted for use in making a variety of articles of manufacture. One useful class of synthetic rubber material compounds includes the nitrile rubber class, which is widely used to make articles such as gloves and oil resistant seals.
Elastomeric articles requiring the highest elongation and greatest ease to stretch, such as surgical or examination gloves, balloons, and condoms have traditionally been made from natural rubber latex. While nitrile rubber products are typically more difficult to stretch, one of the advantages of nitrile rubber over natural rubber latex substrates is that nitrile rubber products do not contain natural latex proteins, which can become a significant allergy issue for some users. Other advantages of nitrile rubber materials over natural rubber latex are that nitrile rubber materials exhibit improved chemical resistance, especially to fatty and oily substances, and improved puncture resistance. Hence, nitrile rubber products have become desirable as a substitute for natural rubber products.
While hospitals, laboratories, or other work environments that may use rubber gloves often want to go “latex free” to better protect their workers, the normally higher cost of nitrile rubber products often limits their ability to make the change. Another hindrance toward making the change is that nitrile rubber gloves traditionally have been stiffer, hence such gloves are less comfortable to wear as compared to similar types of gloves made from natural rubber latex materials.
Currently, it is believed that no nitrile rubber examination gloves are available on the commercial market that exhibit stress-strain and force-strain properties that are close to that of natural rubber latex gloves, not to mention being either similar or the same as natural rubber latex gloves in these terms. Stress-strain properties measure the response to an applied force per unit cross sectional area of the material, while force-strain properties refer to a direct measurement of how a material responds (stretches) in response to an applied force, regardless of the thickness of the material. For instance, natural rubber latex (NRL) examination gloves typically only require a stress of about 4.5 MPa to stretch to an elongation of about 500% over original dimensions. This often is referred to as the glove's 500% modulus. Conventional nitrile rubber examination gloves, on the other hand, typically require more than twice that amount of stress (e.g., about 10 MPa) to achieve the same 500% elongation. In addition, NRL examination gloves typically only require a force of about 1.2 Newtons to stretch to an elongation of 400%. Meanwhile, conventional nitrile rubber examination gloves require a force of about 2.25 Newtons to stretch to an elongation of 400%.
Nitrile rubber, a synthetic polymer often used in emulsion (latex) form to manufacture medical and industrial gloves is a random terpolymer of acrylonitrile, butadiene, and a carboxylic acid such as methacrylic acid. It can be crosslinked by two separate mechanisms to improve its strength and chemical resistance. The first mechanism of crosslinking occurs by ionically bonding carboxylic acid groups together using multivalent metal ions. These ions are typically supplied through addition of zinc oxide to the emulsion. Normally the strength and stiffness/softness of the polymer is very sensitive to this type of crosslinking. The other crosslinking mechanism is a covalent crosslinking of the butadiene segments of the polymer using sulfur and catalysts known as rubber accelerators. This covalent crosslinking is especially important for development of chemical resistance. Gloves are often formed by first placing a coagulant solution, which can contain calcium nitrate, calcium carbonate, or a combination thereof, on ceramic glove molds, then dipping into the nitrile rubber to cause local gelation of the nitrile rubber over the mold surface.
Several prior approaches to softening nitrile rubber articles have involved strongly limiting or completely omitting zinc oxide and other materials capable of ionically crosslinking carboxylated nitrile rubber, such as those described in U.S. Pat. Nos. 6,031,042 and 6,451,893. In addition to not yielding force-strain properties similar to those of comparable natural rubber products as discussed above, this method can result in a material having lower strength, the need for higher curing temperatures, the need for extraordinarily high levels of other chemicals that may cause skin irritation, or processing difficulties such as thickening of the nitrile rubber before dipping.
Other approaches to making a nitrile glove more comfortable, such as those described in U.S. Pat. Nos. 5,014,362 and 6,566,435, have relied on stress relaxation over time and require constantly applied levels of strain to cause the desired relaxation or softening. Such determination measures are difficult to maintain and can be unrealistic in real world practice and use.
As such, a need exists for a nitrile rubber-based article that can successfully combine the benefits of nitrile rubber materials with the greater pliability or softness of natural rubber latex without the need to apply conditions required for softening caused by stress relaxation. There is also a need for a kind of nitrile glove that can incorporate a polymer formulation and product dimensions to simulate the comfort and softness associated with natural rubber latex products, while simultaneously maintaining the protective and non-allergenic properties of nitrile rubber. Desirably, such a glove, when worn, would enable the elastomeric material to exhibit physical strain or stress profiles similar to those of natural rubber, without exposure to natural rubber's associated problems.